Laptop computers now outsell desktops in stores; for the majority of consumers, the smaller devices serve perfectly well as their main computer. That would have been hard to imagine back in the mid-Eighties, when IBM and Apple introduced their first primitive laptops, each weighing in at about twelve pounds. And for long after that, laptops remained either much less functional or much more expensive than the dominant desktops. But now, twenty years later, the laptop has truly become the “desktop replacement.”

A decade or two from now, will there be a laptop replacement? Will an even smaller, more mobile device — perhaps something the size of today’s smartphone — replace the laptop in the lives of consumers? While it’s once again hard to imagine, in fact much of the technology already exists to make it so.

For starters, there is one very obvious problem: the keyboard. As any smartphone owner knows, keyboards keep shrinking — but fingers do not. Typing on a tiny keypad is no way to write even the shortest memo.

The problem, however, is already being addressed by “virtual keyboards”: devices that project the image of a keyboard onto any flat surface. Optical sensors watch where your fingers move on the projection and record each motion as a keystroke. Odd as it sounds, using one of these soon becomes quite natural: the device even produces a soft click each time you touch a key.

Virtual keyboards are already available as accessories: the I-Tech virtual laser keyboard was the first on the market, a $180 box about the size of a small cellphone that connects to your PDA or smartphone via Bluetooth. While the technology isn’t perfect, it’s improving quickly. More importantly, several companies are racing to be the first to have their virtual keyboard technology directly embedded into a cellphone or PDA.

Another possible alternative input for handheld computers is voice recognition. Generally speaking, high-quality voice recognition requires substantial computing power, so handheld devices are still a few years away from being able to take dictation well enough to substitute for a keyboard — but sooner or later they will. The drawback to voice recognition, of course, is social: a planeload of business people simultaneously talking into their handheld computers is not a pleasant thought.

One dark-horse candidate in the field, however, may overcome that prospect. NASA is currently working on a technology called “subvocal speech recognition” in which electrodes taped to your neck recognize the signals your brain sends to your larynx. It turns out that those signals are sent even when you’re just thinking of words. The NASA researchers have conducted Web searches on a computer merely by thinking, but not saying aloud, the commands and search terms. Several laboratories are working on this technology but it’s still very delicate and—as its origin suggests — probably a bit too close to rocket science for near-term practical purposes.

So there are indeed alternatives to the tiny keyboard. How about the display? Smartphone screens are awfully small compared to a laptop, yet they can’t get much bigger and still fit into a purse or jacket pocket.

To solve this, engineers are building microprojectors: today’s familiar LCD projector shrunk to the size of a stick of gum and built directly into smartphones and PDAs. Unlike the virtual keyboard, you can’t buy one of these at retail yet. But several companies, both in the U.S. and abroad, are already well along with microprojectors based on laser or MEMS (micro-electromechanical systems) technology. One version recently shown at conferences projects a bright 11 x 17” image at a distance of about three feet.

There’s another option for viewing: near-eye “wearable displays” — basically, glasses that contain tiny screens placed right in front of your eyes, creating the impression of a full-sized computer screen. Wearable displays aren’t new — you can already buy a number of different models, primarily aimed at video fans or gamers who want to have a big-screen experience without a large monitor. Thus far, however, they haven’t really caught on in a big way, partly due to price — usually well over $500 — and partly because the current models sometimes make users a bit seasick. But once again, multiple manufacturers see a big potential for wearable displays and a variety of new technologies are soon to appear.

Dubious about getting the average businessperson to wear one of these sci-fi looking displays? It actually doesn’t seem that big a step past the glowing Bluetooth headsets that already nest on many commuters’ ears. And upcoming wearable displays will be far smaller, lighter and more stylish than the current vaguely Darth Vader-ish models. Future wearable displays may also have flip-up, flip-down functions, rather like clip-on sunglasses, to make it easier to keep in touch with reality. And ultimately there will probably be ways to superimpose the computer image over a view of the outside world, so you can still keep an eye on your briefcase or purse while you’re writing that memo.

There’s one other challenge in the race to replace laptops: battery life. All the wonders described above will drain yet more power from tiny batteries that are already struggling to keep up with today’s smartphones. One virtual keyboard manufacturer, for example, estimates that when their product is embedded into cellphones and PDAs it will consume 10 percent more electricity. Microprojectors will probably impose even greater demands. Yet lithium ion battery technology — the state of the art — has just about reached its peak, and new approaches to battery technology are still a few years away.

That’s why there’s so much interest worldwide in replacing conventional batteries with tiny fuel cells. Fuel cells promise extended power delivery, and when they do run down, they can be recharged simply by adding a bit of methanol. Like all of the technologies described here, miniature fuel cells are probably still a few years away from the mass market. But then it took the laptop nearly twenty years to surpass the desktop — so there’s still plenty of time left for technologists to deliver the next replacement.